Effect of Triton X-100 as Dispersant on Carbon Black for LiFePO4...

Int. J. Electrochem. Sci., 8 (2013) 6722 - 6733

International Journal of

ELECTROCHEMICAL SCIENCE

www.electrochemsci.org

Effect of Triton X-100 as Dispersant on Carbon Black for

LiFePO4 Cathode

Zhian Zhang, Changming Qu, Tao Zheng, Yanqing Lai, Jie Li*

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China *E-mail: [email protected]

Received: 8 March 2013 / Accepted: 3 April 2013 / Published: 1 May 2013

In this paper, the poly (ethylene glycol) p-isooctyl-phenyl ether (Triton X-100) as a dispersant is used

to pre-disperse the nano-carbon black (SP) in organic N-methyl-pyrrolidone (NMP) solvent. This pre-

dispersed carbon black (SP-T) as conductive agent is applied to LiFePO4 cathodes. The dispersion

property of nano-carbon black with/without Triton X-100 is evaluated using laser granulometry

measurements and rheology test. The LiFePO4 cathodes with pre-dispersed carbon black (SP-T) are

examined by scanning electron microscopy (SEM), cyclic voltammograms (CV), electrochemical

impendence spectroscopy (EIS) and charge/discharge cycling performance test compared with the

LiFePO4 cathode with SP. Results show that the LiFePO4 cathodes with SP-T exhibit improved rate

behavior and better cycle performance than the LiFePO4 cathode with SP. The LiFePO4/SP-T cathode

containing 1.5 wt.% of Triton X-100 presents the best electrochemical property, exhibiting discharge

capacity of 123.9 mAh g-1

after 200 cycles at 1C (93.9% capacity retention) and 101.0 mAh g-1

at 4 C.

Because pre-dispersed carbon black (SP-T) is dispersed homogeneously in the dried composite

electrode to form a better electronic wiring of the active material particles, so as to enhance

electrochemical performance of the cathodes.

Keywords: LiFePO4 cathode, carbon black, Triton X-100, dispersion.

1. INTRODUCTION

Lithium ion battery based on LiFePO4 cathode has been considered as one of the most

promising candidates for large-scale application in electric vehicle, hybrid electric vehicle and smart

grid due to its excellent chemical and thermal stability, good safety, low toxicity and low cost [1-2].

Among the components in the LiFePO4 cathode, the conductive agent plays an important role in

improving cell performance, especially in regards to cycle life [3-4].

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Int. J. Electrochem. Sci., Vol. 8, 2013

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The conductive carbon materials which have good conductivity and electrochemical stability

are widely used as a conductive agent in lithium ion battery, especially nano-conductive carbon

additives, such as acetylene black (AB), Ketjen black (KB), Super P (SP) and carbon nanotubes.

However, it is really difficult to make these kinds of nano-conductive carbon particles disperse

homogeneously into the slurry because they have a tendency to flocculate due to their large surface

area and high oil adsorption, especially when the particulates must be dispersed in highly dense

suspensions of active materials. [5-8] If the conductive particulates are dispersed heterogeneously in

the cathode, not only might the performances of the battery deteriorate, but also production speed,

yield and battery safety could be affect [9].

To overcome this problem, dispersing carbon black before the slurry preparation has been

considered a way to form more homogeneous carbon black distribution within the dried composite

electrode, which could reduce internal resistance and enhance electrochemical performance of cathode

materials [9-10]. Chemical dispersants can be adsorbed on the surface of the nano-particles, and

prevent the particles from agglomerating through electrostatic repulsion or steric effects, thus forming

well-dispersed suspension of nanoparticles [11]. Carbon black can be dispersed by chemical dispersant

in organic medium. Recently, two carboxylic dispersants (EACH, CP) are applied to disperse carbon

black in organic medium, which result in a decrease of powder aggregation and the suspension

viscosity [12]. Tomlinson et al. [13] research on commercial succinimide as a dispersant for

carbonaceous solid substrates in hydrocarbon solvents. Matthew et al. [14] reported three different

dispersants surround carbon black particles and form a colloidal suspension. Moreover, orotan® is an

effective dispersant (polyacrylate dispersant) for dispersing carbon black in distilled water. [15]

However, the application of dispersants for the lithium-ion battery is few, especially for

improvement of carbon black particles dispersion in the dried composite electrode, very little work has

been reported. Triton X-100 was observed to positively affect the dispersion of carbon black for the

aqueous processing of LiFePO4 composite electrodes [16]. But, it is really difficult to completely

remove the moisture remained in the electrode [17]. Chang et al. reported conductive carbon can be

dispersed by HSC-03 in NMP solvent for LiFePO4 cathode [18].

In this work, poly (ethylene glycol) p-isooctyl-phenyl ether (Triton X-100) is used to pre-

disperse the nano-carbon black (SP), which is used commonly in commercial lithium ion batteries as

conductive additive, in organic N-methyl-pyrrolidone (NMP) solvent, because NMP is a most common

industrial organic solvent-based system in battery manufacture. This pre-dispersed carbon black (SP-

T) as conductive agent is applied to LiFePO4 cathodes. We investigate the physical properties of

undispersed SP and pre-dispersed SP, respectively, and study effects of different SP on the

performance of LiFePO4 cathodes for lithium ion batteries. The dispersion properties of SP are

characterized through particle size distribution analysis, scanning electron microscopy and rheology.

The charge/discharge cycling performance, electrochemical redox behavior and impendence of

LiFePO4 cathodes with pre-dispersed or undispersed SP are examined by using a coin-type cell.


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2. EXPERIMENTAL

2.1 Materials and process

The commercial carbon-coated LiFePO4 powders with detailed specifications as follows:

carbon content (2 wt. %), average particle size (1.3 µm), tap density (1.35 g·cm-3

) and BET specific

surface area (13.7 m2·g

-1). Super P with an average particle size of 40 nm was acquired from TIMCAL

Graphite & Carbon, Switzerland. Triton X-100 (Aldrich) MTriton X-100 = 652 g mol−1

was purchased

from Sigma-Aldrich. PVDF (HSV900, Mw=1,000,000) was supplied by Arkema, France.

Pre-dispersion of carbon black: The saturated adsorption amount of Triton X-100 on the SP

surface is the ratio of SP and Triton X-100 for 5:1 through theoretical calculation [19]. Here, the study

to evaluate the more efficient dispersant by optimizing parameters of the mass ratio of SP and Triton

X-100, so we designed the parameters of the mass ratio of SP and Triton X-100 for 10: 1, 20: 3, 5: 1

and 4: 1, respectively. Pre-dispersed suspensions of carbon black were prepared by slowly adding the

required amount of dry SP powder to N-methyl-pyrrolidone (NMP, Merck) with/without a dispersing

agent (Triton X-100). These materials were mixed by a Polytron PT 10-35 homogenizer at 20000 rpm

for 10 min until uniform. The forgoing procedure produce two types of suspensions that SP without

Triton X-100 and SP with the Triton X-100 dispersant. The two suspensions of carbon black with

different dispersion properties were named as SP and SP-T respectively and used as conductive agent

in the following articles.

Preparation of composite electrodes: LiFePO4 and PVDF were added to the two conductive

agent paste (SP: PVDF: LiFePO4 is 1:1:8), which Triton X-100 concentrations corresponds to 0, 1, 1.5,

2, 2.5 wt.% in the dried electrodes. The mixing process was performed in a Polytron PT 10-35

homogenizer at 20000 rpm for 20 min until uniform. The mixed slurry was coated onto aluminum foil

and dried at 110 °C under vacuum for 24 h to remove the residual solvent. Experience procedure is

shown in Fig. 1.

Figure 1. Experiment procedure


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2.2 Characterization of the suspensions

Particles size distribution in dilute medium was measured by low angle laser light scattering

using a Mastersizer S laser granulometer (Malvern). A drop of SP/SP-T suspension was diluted in 100

mL of NMP before measurement. The rheological behavior of the SP / SP-T suspension was

determined at 25 ℃ using a controlled-stress rheometer (AR1000, TA Instruments Ltd., UK) with a 50

mm diameter plane-and-plane geometry. The classical solvent trap accessory was used to prevent the

sample from drying during the measurement. Before the measurement, the sample was need to pre-

shear at the rate 1000 s−1

for 60 s, and then to let it rest for 600 s. It ensures that all samples have the

same mechanical history. Viscosity or shear stress was then measured as a function of the shear rate

from 1000 to 0.1 s−1

to determine viscosity.

2.3 Characterization of the electrodes

Morphology of the electrodes was conducted by SEM (Quanta-200, FEI). The electrochemical

performances of the LiFePO4 cathodes were evaluated in CR2025-size button LiFePO4/Li half cells,

which were assembled by using Celgard® 2400 membrane as the separator and filled with 85 μL of

liquid electrolyte. The lithium electrode in this half cell was used as the counter electrode as well as the

reference electrode and the electrolyte was 1 mol/L LiPF6 dissolved in EC/DMC/EMC 1:1:1 by mass

co-solvent, which was purchased from Zhangjiagang Guotai-Huarong New Chemical Materials Co.,

Ltd.,China. All the cells were assembled in an argon-filled glove box (Universal 2440/750, Mikrouna

Mech. Tech. Co., Ltd., water content:


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[20] Whereas the discharge capacity of LiFePO4/SP-T cathode (1.5 wt.% of Triton X-100) is 140.2

mAh·g-1

for the initial cycle and a high reversible capacity of 127.9 mAh·g-1

is retained after 200

cycles with the capacity retention ratio of 93.9%. These results confirm that the LiFePO4/SP-T cathode

showing a great improvement in cyclability.

Figure 2. Cycle performance of cathodes with different Triton X-100 contents at 1 C between 2.5 V

and 4.2 V. The inset figure shows the comparison of LiFePO4/SP cathode and LiFePO4/SP-T

cathode containing 1.5 wt.% of Triton X-100

The rate capability of LiFePO4/SP cathodes with various Triton X-100 contents is shown in

Fig. 3. The electrodes have the similar discharge capacities at 0.1 C. However, with the increase of

discharge–charge current rate, electrochemical performance of the electrode are influenced by the

contents of Triton X-100, i.e. the discharge capacity increases until 1.5 wt.% of Triton X-100. The

discharge capacity for LiFePO4/SP-T cathode containing 1.5 wt.% of Triton X-100 is 101 mAh g-1

but

LiFePO4/SP cathode is 88 mAh g-1

at 4 C. Compared with the LiFePO4/SP cathode, LiFePO4/SP-T

cathodes show a better discharge capacity, especially in the case of high rate. It is likely that the

variation of the discharge capacity for these electrodes is due to the difference in the electronic wiring

of the active particles. The improvement of rate capability for LiFePO4/SP-T cathodes can be

attributed to that the SP is more homogeneously dispersed, improving the electrical contact between

the SP and LiFePO4 nano-particles and can obtain the lower potential polarization of the cathode

during the charge and discharge processes.


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Figure 3. Rate capability of cathodes with different Triton X-100 contents at 0.1~4 C

To better characterize the capacity fading mechanisms of the LiFePO4 cathodes, EIS method

was used for measuring the cell containing the LiFePO4 cathode, which can help to clarify the ohmic

resistance and polarization resistance behaviors coincident with capacity loss. Fig. 4 shows Nyquist

plots obtained from the LiFePO4/SP-T and the LiFePO4/SP cells after 200 cycles test. The impedances

of the cells after cycle test were measured at a fully charged state and analyzed by Zview software. In

general, a high frequency pronounced semicircle is related to the resistance of charge transfer reaction

resistance (Rct), which represents the charge transfer capacity in the electrode [21-22]. It can be seen

from Fig. 4 that the Rct of LiFePO4/SP cell is maximum. Rct of LiFePO4/SP-T cells reduced

significantly and with Triton X-100 concentrations increased, Rct of LiFePO4/SP-T cells was decreased

firstly then increased, which Triton X-100 content corresponds to 1.5% is minimum. This variation

trend of EIS is consistent with the cycle performances of cells in Fig. 3.

The change in impedance is a main factor which affects the cycling and rate performance of

batteries. After adding the right amount of Triton X-100, the agglomeration of SP has been improved.

The SP could be dispersed in the surrounding of the LiFePO4 particles uniformly to form good

conductive channels, so that reduced the polarization of battery during the charge and discharge

processes, thus the reversibility of battery and electrochemical performances are improved. However,

when adding too more Triton X-100, the dispersants were adsorbed on the SP surface and their tail-end

long chain would produce a "bridged" phenomenon. That could cause soft agglomeration of the SP

particles. And the insulativity of Triton X-100 may reduce the charge transfer capability of the SP and

LiFePO4 particles, which lead to the impedance increased. Therefore, the trend of Rct was decreases

firstly and then increases. Based on the above data, the electrochemical performance of the cells when

the cathode containing 1.5 wt.% of Triton X-100 is best. So that we mainly compared the difference

between these two samples (LiFePO4/SP and LiFePO4/SP-T containing 1.5 wt.% of Triton X-100) in

the following article.


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Figure 4. A. C. impedance plots of cathodes with different Triton X-100 contents after 200 cycles

The cyclic voltammograms of the LiFePO4/SP and LiFePO4/SP-T cathode containing 1.5 wt.%

of Triton X-100 are shown in Fig. 5, respectively. A pair of redox peaks appears in the CV curves is

typical redox peaks of LiFePO4 electrode [23]. It is clear that the oxidation peaks of LiFePO4/SP-T

cathode shift to the left, the reduction peaks shift to the right, the voltage difference between the redox

peaks decrease from 0.486 V to 0.360 V, and the redox peaks are much sharper than that of the

LiFePO4/SP cathode. These confirm that the redox reaction rate of the LiFePO4/SP-T cathode is faster,

and indicates the weakening of polarization in the cell, better kinetics characteristics and better

reversibility of LiFePO4/SP-T cathode. Since both cathodes are fabricated identically except for the

inclusion of Triton X-100, this confirms that by using Triton X-100 during cathode fabrication

improves the dispersion of the SP, presumably because the electron transfer rate is increased by more

uniform conductive channels. The curves of sweeps 1 and 2 for the LiFePO4/SP-T cathode are very

similar in shape. It should be noted that the dispersant Triton X-100 is electrochemically stable over

the 2.5-4.5 V voltage range.

The effect of Triton X-100 on SP dispersion is investigated further by scanning electron

microscopy imaging of the LiFePO4 cathode surface. Fig. 6 (a) and (b) show microstructures of the top

surfaces of LiFePO4 electrode prepared with SP and SP-T, respectively. It is obvious that SP formed

heavy agglomeration itself, which cannot be well scattered on the LiFePO4 surface to form an effective

contact in Fig. 6 (a). In contrast, the Fig. 6 (b) shows the LiFePO4 cathode prepared with SP-T is seen

to possess better uniformity and smaller particle size of SP-T surrounding the LiFePO4 particles which

suggests an improved electrical contact between the SP-T and LiFePO4 particles. Compared to the

cathode with SP, the particles of the cathode with SP-T could form the better conductive channels that

weakening the polarization. So the rate capability of the battery could be enhanced.


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Figure 5. Cyclic voltammograms of LiFePO4/SP and LiFePO4/SP-T cathodes containing 1.5% Triton

X-100: Sweep rate is 0.1 mV·s-1

.

Figure 6. SEM images of different LiFePO4 cathodes before charge, (a) LiFePO4/SP and (b)

LiFePO4/SP-T containing 1.5% Triton X-100

3.2 Triton X-100 dispersion effect tests

Particle size distribution of SP and SP-T in the NMP solvent was measured in order to evaluate

their agglomeration state, and the results were shown in Fig. 7. We can see that the SP apparent

particle size in the NMP solution (D50) is about 12.12 μm, larger than that of SP-T (6.60 μm). In both

cases, the suspended SP particles are quite larger than the initial 40 nm of the as-supplied SP powder,

demonstrating a basic flocculation tendency of the NMP-suspended SP particulates because the poor

compatibility between hydrophobic carbon black and NMP (polar energy ∼12.3 MPa1/2) [24]. But the

particle size of SP-T is significantly smaller than the SP, besides the particle size distribution is

significantly narrower, which indicate that the particle size is more uniform. Particle size and


6730

distribution are affected by the dispersion properties of SP in NMP. Well-dispersed SP results in the

particle size distributions of SP shift to the left, i.e., toward smaller particle diameters. Improvement of

the SP-T dispersion in the electrode slurries results in more homogeneous SP distribution within the

dried composite electrode and better electronic wiring of the active material particles, leading to higher

discharge capacity at high rates.

Figure 7. Particle size distribution of carbon black

Figure 8. Schematic illustration of dispersion principle

These improved dispersion properties of SP-T originate from Triton X-100 has a favorable

conformation for dispersion of particles due to steric effects. The isooctyl chain on the benzene ring

could be adsorbed on SP, whereas the benzene ring contributes greatly to the adsorption to the

graphitic surface because of π-π stacking type interaction [11, 19]. The lyophilic group (PEO) facing


6731

toward the medium provides effective steric stabilization to overcome the large van der Waals forces

among the SP particles, thereby improving the dispersion effect of SP. Schematic illustration of

dispersion principle is present in Fig. 8.

Rheological behavior of SP suspension and SP-T suspension (SP: Triton X-100 is 20: 3) were

characterized, and the results are shown in Fig. 9. All suspensions show shear thinning behaviors,

showing linear dependence of the viscosity on the shear rate, i.e. a decrease of the viscosity with

increasing shear rate. This is typical of flocculated dispersions where the presence of weak attractive

forces between the particles leads to the formation of clusters of particles. It is also noticeable that the

addition of Triton X-100 significantly decreased the viscosity of the SP-T suspension, which resulted

from the improvement of dispersion properties of SP-T. This is well-consistent with the results for

particle size distribution (Fig. 7). Therefore, it is clear that the addition of Triton X-100 contributes to

the improvement of dispersion properties of SP-T through steric stabilization.

Figure 9. Relative viscosity as a function of shear rate for SP and SP-T (SP: Triton X-100 is 20: 3)

4. CONCLUSIONS

In this paper, we have clearly demonstrated that nano-conductive carbon agent can be pre-

dispersed with Triton X-100 as dispersant in organic N-methyl-pyrrolidone (NMP) solvent system and

the dispersion properties of nano-conductive carbon agent have a great impact on the electrochemical

performance of the electrodes

When the LiFePO4/SP-T cathode containing 1.5 wt.% of Triton X-100, the SP-T had a

narrower size range than SP and the LiFePO4/ SP-T cathode has more homogeneous microstructures

compared to the LiFePO4/SP cathode. Improvement of the SP-T dispersion in the electrode slurries


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results in more homogeneous SP-T distribution within the dried composite electrode and better

electronic wiring of the active material particles, leading to better electrochemical performance.

A dispersant is needed to disperse the nano-conductive carbon agent (SP) and obtain a

satisfactory suspension processing of a LiFePO4 composite electrode for lithium ion battery. The poly

(ethylene glycol) p-isooctyl-phenyl ether (Triton X-100) is an efficient non-ionic surfactant which can

be used for the purpose. We believe that such methodology will have certain advantage for the

development of the lithium ion battery technology in the future. Our future work will continue to

explore alternative formulae for Triton X-100, as well as various combinations of alternative SP

materials.

ACKNOWLEDGMENTS

The authors thank the financial support of the National Key Technology Research and Development

Program of China (2007BAE12B01) and the National Natural Science Foundation of China

(20803095). We also thank the support of the Engineering Research Center of Advanced Battery

Materials, the Ministry of Education, China.

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